Many machines include a power system operable to propel the machine by transmitting power from a prime mover (such as an engine or electric motor) to propulsion devices (such as wheels or tracks) through a multiple-ratio transmission. Such a multiple-ratio transmission can transmit power at any of a number of transmission drive ratios, or speed ratios between the input and the output of the multiple-ratio transmission. As a result, there often exist multiple alternative combinations of prime-mover operating speed and transmission drive ratio that could be used to propel the machine at any particular speed. The prime-mover operating speed and transmission drive ratio used to propel the machine at any given speed may affect various aspects of the performance of the machine, including the energy efficiency of the power system.
U.S. Patent Application Publication No. 2004/0254047 to Frank et al. (“the '047 application”) discloses controlling an engine and an associated continuously variable transmission in a manner to maintain operation of the engine constantly on a speed/power curve where the engine has its maximum energy efficiency. The control system disclosed in the '047 application determines either a power requirement or a torque requirement based on operator input received via an accelerator pedal. In response to changes in the determined power or torque requirement, the control system adjusts the engine operating speed and torque, as well as the transmission drive ratio, while maintaining operation of the engine constantly on the speed/power curve where it has its maximum energy efficiency.
Although the control method of the '047 application discloses controlling its engine and continuously variable transmission to maintain the engine on the speed/power curve where the engine has its maximum energy efficiency, certain disadvantages persist. For example, in some applications and/or circumstances, other considerations may make it undesirable to constrain operation of the engine exclusively to the speed/power curve where it has its highest energy efficiency. Some circumstances may warrant operating the engine and continuously variable transmission at a different operating point in order to enhance the ability to effectively respond to any increase in power demands that may occur.